1. Field of the Invention
This invention relates to halide glass compositions and more particularly to halozirconate glass compositions which, have good properties as hosts for rare earth elements as lasing dopants.
2. Related Art
It has long be recognised that the rare earth elements display fluorescence and this fluorescence can be utilised in the form of lasing either for the generation of optical signals or for the amplification of optical signals. Usually the lasing species is a trivalent ion of a rare earth element. In particular the trivalent ion Pr.sup.3+ (praseodymium) constitutes a lasing species for providing radiation at 1300nm. This property is of interest because optical telecommunications uses signals at 1300nm and the ion Pr.sup.3+ is capable of amplifying such signals by laser action. It will be apparent that, in order to take advantage of this property, it is necessary to provide the active species in a suitable waveguiding structure, eg. a fibre waveguide.
While the element Pr is of particular interest for telecommunications it should be recognised that all the rare earth elements are capable of lasing at a variety of different wavelengths for a variety of different purposes. In other words the lasing properties extend throughout the group of rare elements and there is, therefore, general interest in providing all of the rare earth elements as lasing species in suitable host glasses.
The halide, eg fluoride, glasses have been recognised since 1978 and a wide range of compositions have been reported and their properties studied. It has been recognised that the halide glasses form good hosts for the rare earth elements as lasing species but the identification and selection of compositions having favourable properties remains difficult. In particular the prior art has failed to identify the glass compositions capable of lasing at 1300 nm with sufficient efficiency for use in telecommunications networks. This invention relates to compositions which have good properties. It is now convenient to discuss the properties of the class required in a lasing device such as a fibre amplifier. These properties will be considered under three different headings.
General Glass Properties
It is important that all glasses shall remain in the glass state, ie. they shall not devirtify under condition of use. It is also important that the glasses shall not be subject to crystallisation which might be considered as incipient devitrification. In addition it is also necessary that the compositions shall be suitable for use in glass forming and further processing. In particular it is necessary that a composition be stable in the melt, that it shall be capable of withstanding practical cooling rates and the conditions necessary for fibre forming, eg. during the pulling of a fibre preform into a fibre. It will also be apparent that chemical stability of the various glass components is important, eg it is desirable to avoid water soluble ingredients and, even more important, to avoid hygroscopic ingredients.
Attenuation
Lasing devices usually include waveguiding structures and it is clearly important to avoid unnecessary attenuation of either the signal wavelength or the pump wavelength. The requirement for low attenuation means that it is desirable to avoid components which have unnecessarily high absorptions at wavelengths of interest. It is also necessary to avoid scatter which emphasises some of the fundamental glass properties, ie. that the glass shall not form crystals even on a small scale.
Host Properties
It also appears that there is interaction between the host glass and the lasing species. For example, the lasing species may undergo what is often called "non-radiative decay". This implies that the lasing species looses energy other than by the intended lasing transitions. Non-radiative decay represents a loss of energy and it is, therefore, an undesirable effect. It appears that the host glass may participate in non-radiative decay either in the sense that it may assist this undesired effect or help to inhibit it. Nevertheless, whatever the reason, it is established that the host glass can affect the efficiency of the lasing process and it is desirable to select the host so as to achieve good lasing efficiencies.
The hosting properties of the glass appear to have substantial effects upon the efficiency of a laser, eg. the ratio of signal power output to pump power input. This efficiency is of substantial importance in telecommunications because it may define the available gain of an amplifier. In experimental work, it is often convenient to utilise the lifetime of the excited state as a measure of the efficiency; the two quantities can be regarded as proportional to one another. In some theoretical papers it is considered that the multi-phonon absorption of the host affects the lifetime of the excited state and hence the efficiency of lasers based thereon.
It is important to recognise that the selection of a lasing composition, and especially the host glass, must take into account all of these features. Thus it is not necessarily appropriate to select ingredients solely on the basis of their effect upon the lasing performance if these components are liable to give rise to glass instability and high attenuations (which high attenuations may be the result of glass instability). In other words, selecting on the basis of one desirable feature is unlikely to produce acceptable results if this selection is accompanied by adverse effects.
It has been mentioned that the prior art has disclosed and evaluated a very wide range of different halide (fluoride) glasses. This range includes a well recognised group usually known as fluorozirconates. This sub-group of fluoride glasses has been recognised because its members perform well in respect of all of the above features. The chemical composition of the fluorozirconate glasses will now be described.
The major component is ZrF4 which usually constitutes about 40-65 mole % of the total composition. In some variants the content of ZrF4 is reduced in order to adjust the refractive index, eg. by incorporating PbF2 or HfF2. (Refractive index adjustment is important in the design of waveguiding structures). A fluorozirconate composition usually contains about 10-39, eg. 15-25, mole % of an alkali metal fluoride, usually NaF. In addition, the composition often contains a substantial amount, eg. 10-mole % of BaF2 with smaller amounts, eg. 2-6 mole %, of LaF3 and AlF3. It is emphasised that the halide content of a fluorozirconate glass is entirely fluoride. In the case of a lasing composition, the fluorozirconate host will also contain up to 4 wt % of the cation of a rare earth metal, eg 0.001 to 0.1 wt % (ie 10-1000 ppm. wt) of Pr.sup.3+.